Metallization of solar cells using metal foils

ABSTRACT

A solar cell structure includes P-type and N-type doped regions. A dielectric spacer is formed on a surface of the solar cell structure. A metal layer is formed on the dielectric spacer and on the surface of the solar cell structure that is exposed by the dielectric spacer. A metal foil is placed on the metal layer. A laser beam is used to weld the metal foil to the metal layer. A laser beam is also used to pattern the metal foil. The laser beam ablates portions of the metal foil and the metal layer that are over the dielectric spacer. The laser ablation of the metal foil cuts the metal foil into separate P-type and N-type metal fingers.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cells. More particularly, embodiments of the subject matter relateto solar cell fabrication processes and structures.

BACKGROUND

Solar cells are well known devices for converting solar radiation toelectrical energy. A solar cell has a front side that faces the sunduring normal operation to collect solar radiation and a backsideopposite the front side. Solar radiation impinging on the solar cellcreates electrical charges that may be harnessed to power an externalelectrical circuit, such as a load. The external electrical circuit mayreceive electrical current from the solar cell by way of metal fingersthat are connected to doped regions of the solar cell.

BRIEF SUMMARY

In one embodiment, a dielectric spacer is formed on a surface of a solarcell structure. A metal layer is formed on the dielectric spacer and onthe surface of the solar cell structure that is exposed by thedielectric spacer. A metal foil is placed on the metal layer. A laserbeam is used to weld the metal foil to the metal layer. A laser beam isalso used to pattern the metal foil. The laser beam ablates portions ofthe metal foil and the metal layer that are over the dielectric spacer.The laser ablation of the metal foil cuts the metal foil into separateP-type and N-type metal fingers.

These and other features of the present disclosure will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures. The drawings are notdrawn to scale.

FIGS. 1-7 are cross-sectional views that schematically illustrate amethod of fabricating a solar cell in accordance with an embodiment ofthe present disclosure.

FIG. 8 is a plan view of an unpatterned metal foil in accordance with anembodiment of the present disclosure.

FIG. 9 is a plan view of the metal foil of FIG. 8 after patterning, inaccordance with an embodiment of the present disclosure.

FIG. 10 is a flow diagram of a method of fabricating a solar cell inaccordance with an embodiment of the present disclosure.

FIGS. 11 and 12 are cross-sectional views that schematically illustratepatterning of a metal foil at the module level, in accordance with anembodiment of the present disclosure.

FIGS. 13 and 14 are cross-sectional views that schematically illustrateuse of a metal foil with a patterned metal layer, in accordance with anembodiment of the present disclosure.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

In the present disclosure, numerous specific details are provided, suchas examples of structures and methods, to provide a thoroughunderstanding of embodiments. Persons of ordinary skill in the art willrecognize, however, that the embodiments can be practiced without one ormore of the specific details. In other instances, well-known details arenot shown or described to avoid obscuring aspects of the embodiments.

FIGS. 1-7 are cross-sectional views that schematically illustrate amethod of fabricating a solar cell in accordance with an embodiment ofthe present disclosure. The solar cell being fabricated is an all backcontact solar cell in that the N-type and P-type doped regions and themetal fingers coupled to the N-type and P-type doped regions are on thebackside of the solar cell.

Referring first to FIG. 1, there is shown a solar cell structure 100 inaccordance with an embodiment of the present disclosure. In the exampleof FIG. 1, the solar cell structure 100 comprises a plurality ofalternating N-type doped regions and P-type doped regions that may beformed within a solar cell substrate 101 or external to the solar cellsubstrate 101. For example, the N-type and P-type doped regions may beformed by diffusing N-type and P-type dopants, respectively, into thesolar cell substrate 101. In another example, the N-type and P-typedoped regions are formed in a separate layer of material, such aspolysilicon, that is formed on the solar cell substrate 101. In thatexample, N-type and P-type dopants are diffused into the polysilicon(which may or may not be trenched) to form N-type and P-type dopedregions in the polysilicon, instead of in the solar cell substrate 101.The solar cell substrate 101 may comprise a monocrystalline siliconwafer, for example.

In the example of FIG. 1, the labels “N” and “P” schematically representthe N-type and P-type doped regions or electrical connections to theN-type and P-type doped regions. More particularly, the labels “N”schematically represent exposed N-type doped regions or exposed metalconnections to the N-type doped regions. Similarly, the labels “P”schematically represent exposed P-type doped regions or exposed metalconnections to the P-type doped regions. The solar cell structure 100may thus represent the structure of a solar cell being fabricated aftercontact holes to the N-type and P-type doped regions have been formed,but before the metallization process to form metal contact fingers tothe N-type and P-type doped regions.

In the example of FIG. 1, the N-type and P-type doped regions are on thebackside of the solar cell structure 100. The backside of the solar cellstructure 100 is opposite the front side, which is directed towards thesun to collect solar radiation during normal operation.

Referring next to FIG. 2, a plurality of dielectric spacers 103 areformed on the surface of the solar cell structure 100. In the example ofFIG. 2, a dielectric spacer 103 is formed on a region on the surface ofthe solar cell structure 100 that is over an interface between adjacentP-type and N-type doped regions. As can be appreciated, the dielectricspacers 103 may be also formed on other regions depending on theparticulars of the solar cell structure 100.

In one embodiment, the dielectric spacers 103 are printed on the solarcell structure 100 by screen printing. The dielectric spacers 103 mayalso be formed using other dielectric formation processes, including byspin coating and by deposition (e.g., chemical vapor deposition)followed by patterning (e.g., masking and etching). The dielectricspacers 103 may comprise a dielectric material with optical absorbers, afireable dieletric, etc. As a particular example, the dielectric spacers103 may comprise polyimide (e.g., with titanium oxide filters) that isscreen printed on the solar cell structure 100 to a thickness of 1-10microns. Generally speaking, the dielectric spacers 103 may beconfigured to have a thickness and composition that will block (e.g., byabsorption or reflection) the laser beam employed in the patterning ofthe metal foil 105 (see FIG. 5), and are compatible with the processemployed to form the overlying metal layer (e.g., FIG. 3, metal layer104).

In the example of FIG. 2, each of the dielectric spacers 103 is formedover an N-type doped region and a P-type doped region of the solar cellstructure 100. As will be more apparent below, in a subsequentmetallization process, a metal foil is patterned using a laser while themetal foil is on the solar cell structure 100. The dielectric spacers103 advantageously block laser beams that may penetrate to the solarcell structure 100 during patterning of the metal foil 105.

As shown in FIG. 3, a metal layer 104 is formed on the solar cellstructure 100. The metal layer 104 provides electrical connection to theN-type and P-type doped regions for the subsequently formed metalfingers. In one embodiment, the metal layer 104 comprises a continuousblanket metal coating that is conformal over the dielectric spacers 103.For example, the metal layer 104 may comprise aluminum that is formed onthe dielectric spacers 103, the N-type doped regions, and the P-typedoped regions by sputtering, deposition, or some other process to athickness of 100 Angstroms to 5 microns (e.g., 0.3 micron to 1 micron).Generally speaking, the metal layer 104 comprises a material that can bebonded to the metal foil 105. For example, the metal layer 104 maycomprise aluminum to facilitate welding to an aluminum metal foil 105.The metal layer 104 still electrically connects the N-type doped regionsto the P-type doped regions in FIG. 3. The metal layer 104 issubsequently patterned to separate the N-type doped regions from theP-type doped regions during patterning of the metal foil 105.

Referring next to FIG. 4, the metal foil 105 is roughly located abovethe solar cell structure 100. The metal foil 105 is a “metal foil” inthat it comprises a pre-fabricated thin sheet of metal. FIG. 8 is a planview of the metal foil 105 at this stage of the fabrication process. Asshown in FIG. 8, the metal foil 105 is unpatterned. As will be moreapparent below, the metal foil 105 is subsequently patterned to formmetal fingers of the solar cell after the metal foil 105 has been fittedto the metal layer 104.

Continuing in FIG. 5, the metal foil 105 is placed on the solar cellstructure 100. Unlike metal that is deposited or coated on the solarcell structure 100, the metal foil 105 is a pre-fabricated sheet. In oneembodiment, the metal foil 105 comprises a sheet of aluminum. The metalfoil 105 is placed on the solar cell structure 100 in that it is notformed on the solar cell structure 100. In one embodiment, the metalfoil 105 is placed on the solar cell structure 100 by fitting to themetal layer 104. The fitting process may include pressing the metal foil105 to the metal layer 104 such that the metal foil 105 makes intimatecontact with the metal layer 104. The fitting process may result in themetal foil 105 being conformal over features (e.g., bumps) of the metallayer 104. Vacuum may be used to press the metal foil 105 against themetal layer 104 to obtain a gap of less than 10 microns between themduring welding. A pressure plate may also be used to press the metalfoil 105 against the metal layer 104 during welding; the pressure plateis removed for laser ablation.

FIG. 6 shows the solar cell structure 100 after the metal foil 105 iselectrically bonded to the metal layer 104. In the example of FIG. 6,the metal foil 105 is welded to the metal layer 104 by directing a laserbeam on the metal foil 105 while the metal foil 105 is pressed againstthe metal layer 104. The laser welding process creates weld joints 106that electrically bond the metal foil 105 to the metal layer 104.Because the metal foil 105 is unpatterned at this stage of thefabrication process, the metal foil 105 still electrically connects theN-type and P-type doped regions of the solar cell structure 100.

Continuing in FIG. 7, the metal foil 105 is patterned to form metalfingers 108 and 109. In one embodiment, the metal foil 105 is patternedby ablating portions of the metal foil 105 and metal layer 104 that areover the dielectric spacers 103. The metal foil 105 and the metal layer104 may be ablated using a laser beam. The laser ablation process maycut (see 107) the metal foil 105 into at least two separate pieces, withone piece being a metal finger 108 that is electrically connected to theN-type doped regions and another piece being a metal finger 109 that iselectrically connected to the P-type doped regions. The laser ablationprocess breaks the electrical connection of the N-type and P-type dopedregions through the metal layer 104 and the metal foil 105. The metalfoil 105 and the metal layer 104 are thus patterned in the same step,advantageously reducing fabrication cost.

FIG. 9 is a plan view of the patterned metal foil 105 of FIG. 7 inaccordance with an embodiment of the present disclosure. FIG. 9 showsthat the cut 107 physically separates the metal finger 108 from themetal finger 109. In the example of FIG. 9, the metal foil 105 ispatterned to form interdigitated metal fingers 108 and 109. Other metalfinger designs may also be employed depending on the solar cell.

Returning to FIG. 7, the laser ablation process uses a laser beam thatcuts the metal foil 105 and the metal layer 104 all the way through.Depending on the process window of the laser ablation process, the laserbeam may also cut portions of, but not through, the dielectric spacer103. The dielectric spacers 103 advantageously block laser beams thatmay otherwise reach and damage the solar cell structure 100. Thedielectric spacers 103 also advantageously protect the solar cellstructure 100 from mechanical damage, such as during fitting of themetal foil 105 to the metal layer 104. The dielectric spacers 103 can beleft in the completed solar cell, so their use does not necessarilyinvolve an additional removal step after patterning of the metal foil105.

In light of the foregoing, one of ordinary skill in the art willappreciate that embodiments of the present disclosure provide additionaladvantages heretofore unrealized. Use of metal foils to form metalfingers is relatively cost-effective compared to metallization processesthat involve deposition or plating of the metal fingers. The dielectricspacers 103 allow for a laser welding process and a laser ablationprocess to be performed in-situ, i.e., one after another in the sameprocessing station. The dielectric spacers 103 also enable use of alaser beam to pattern the metal foil 105 while the metal foil 105 is onthe solar cell structure 100. As can be appreciated, placing andaligning a sheet of metal foil is much easier compared to placing andaligning separate strips of metal fingers with accuracy in the order ofmicrons. Unlike etching and other chemical based patterning processes,patterning the metal foil 105 using a laser minimizes the amount ofresidue that may form on the solar cell being fabricated.

It is to be further noted that in the example of FIG. 9, the metal layer104 is patterned simultaneously with the metal foil 105. Thisadvantageously eliminates extraneous steps to pattern the metal layer104 to separate the P-type and N-type doped regions before laser weldingand ablation.

FIG. 10 shows a flow diagram of a method of fabricating a solar cell inaccordance with an embodiment of the present disclosure. The method ofFIG. 10 may be performed on a solar cell structure with N-type andP-type doped regions. The method of FIG. 10 may be performed at the celllevel during fabrication of the solar cell or at the module level whenthe solar cell is connected and packaged with other solar cells. Notethat in various embodiments, the method of FIG. 10 may includeadditional or fewer blocks than illustrated.

In the method of FIG. 10, a plurality of dielectric spacers is formed ona surface of the solar cell structure (step 201). Each of the dielectricspacers may be formed over an N-type doped region and a P-type dopedregion of the solar cell structure. The dielectric spacers may be formedby screen printing, spin coating, or by deposition and patterning, forexample. A metal layer is thereafter formed on the dielectric spacersand on the surface of the solar cell structure that is exposed betweenthe dielectric spacers (step 202). In one embodiment, the metal layer isa continuous and conformal layer that is formed by blanket deposition. Ametal foil is fitted to the metal layer (step 203). In one embodiment,the metal foil is welded to the metal layer using a laser beam (step204). It is to be noted that non-laser based welding techniques may alsobe employed to weld the metal foil to the metal layer. A laser beam mayalso be used to ablate portions of the metal foil and the metal layerthat are over the dielectric spacer (step 205). The laser ablationprocess patterns the metal foil into separate metal fingers, andpatterns the metal layer to separate the P-type and N-type dopedregions.

The patterning of the metal foil 105 may be performed at the modulelevel when the solar cell being manufactured is packaged with othersolar cells. In that example, the metal foil 105 may be fitted to metallayers 104 of a plurality of solar cell structures 100. This isschematically illustrated in FIG. 11, where a metal foil 105A is fittedto metal layers 104 of two or more solar cell structures 100. The metalfoil 105A is the same as the previously discussed metal foil 105 exceptthat the metal foil 105A spans more than one solar cell structure 100.As shown in FIG. 12, the metal foil 105A may be patterned by laserablation while on the solar cell structures 100. The laser ablationprocess may pattern the metal foil 105A into metal fingers 108 and 109as previously discussed. The metal foil 105A may be cut after patterningto physically separate the solar cell structures 100. After patterning,portions of the metal foil 105A may also be left in place to stringtogether adjacent solar cell structures 100.

In one embodiment, the laser ablation of the metal foil 105A leaves aconnection between opposite type metal fingers of adjacent solar cellstructures 100. This is schematically illustrated in the example of FIG.12, where the metal foil 105 is patterned such that a P-type metalfinger 109 of one solar cell structure 100 is left connected to theN-type metal finger 108 of an adjacent solar cell structure 100, therebyelectrically connecting the solar cell structures 100 in series. Thisadvantageously saves fabrication steps at the module level because thepatterning of the metal foil 105A may be combined with the stringing ofthe solar cell structures 100.

As explained, the metal layer 104 may be formed as a blanket layer ofmetal that electrically connects the P-type and N-type doped regions andthereafter patterned to separate the P-type and N-type doped regionsduring patterning of the metal foil 105. In other embodiments, dependingon the particulars of the fabrication process, the metal layer 104 maybe patterned before laser welding and ablation. This is schematicallyillustrated in FIG. 13, where the metal layer 104 is formed on theP-type and N-type doped regions without electrically connecting them.For example, the metal layer 104 may be deposited by blanket depositionover the dielectric spacers 103, the N-type doped regions, and theP-type doped regions, and then patterned (e.g., by masking and etching)to separate the N-type doped regions from the P-type doped regions asshown in FIG. 13. The metal foil 105 may then be placed on the patternedmetal layer 104 and dielectric spacers 103, laser welded to the metallayer 104, and patterned by laser ablation as previously described. FIG.14 schematically shows the N-type metal fingers 108 and P-type metalfingers 109 after the laser ablation process in that embodiment. Thelaser ablation process cuts through the metal foil 105 but stops at thedielectric spacers 103.

Methods and structures for fabricating solar cells have been disclosed.While specific embodiments have been provided, it is to be understoodthat these embodiments are for illustration purposes and not limiting.Many additional embodiments will be apparent to persons of ordinaryskill in the art reading this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

What is claimed is:
 1. A method of fabricating a solar cell, the methodcomprising: forming a dielectric spacer on a surface of a solar cellstructure; forming a metal layer on the dielectric spacer, an N-typedoped region, and a P-type doped region, wherein the metal layerelectrically connects the N-type doped region to the P-type dopedregion; placing a metal foil on the metal layer; and after placing themetal foil on the metal layer, patterning the metal foil, whereinpatterning the metal foil includes removing portions of the metal foiland the metal layer that are over the dielectric spacer.
 2. The methodof claim 1, wherein patterning the metal foil comprises directing alaser beam on the metal foil to ablate the metal foil.
 3. The method ofclaim 2, wherein the laser beam also ablates at least a portion of thedielectric spacer under the metal layer.
 4. The method of claim 1,further comprising: welding the metal foil to the metal layer.
 5. Themethod of claim 1, further comprising: welding the metal foil to themetal layer by directing a laser beam on the metal foil.
 6. The methodof claim 1, wherein the metal layer is formed on the dielectric spacerby blanket deposition.
 7. The method of claim 1, wherein the metal foilis patterned into a P-type metal finger and an N-type metal finger, andthe P-type metal finger is physically and electrically separate from theN-type metal finger.
 8. The method of claim 1, wherein the metal foil isplaced on the metal layer of the solar cell structure and another metallayer of another solar cell structure.
 9. The method of claim 8, whereinpatterning the metal foil includes leaving an electrical connectionbetween metal fingers of the solar cell structure and the other solarcell structure.